Ethylene homopolymers and copolymers having high densities and high melt indices can be produced as described in European patent publication Nos. 0012147 and 0012148. In producing such polymers, it is necessary to employ high polymerization temperatures and to maintain a high hydrogen/ethylene ratio in the reactor in order to obtain polymers having melt indices greater than 1.0 g/10 minutes. Unfortunately, however, an increase in polymerization temperatures above 90.degree. C. and the use of high hydrogen/ethylene ratios have both been found to cause a decrease in the activity of the catalysts employed in these processes. Thus, the desired polymers can only be obtained at the expense of decreased catalytic activity.
Another characteristic of the process described in European patent publication Nos. 0012147 and 0012148 is that a certain undersirable amount of ethylene hydrogenation takes place during polymerization. This ethylene hydrogenation is particularly pronounced when temperatures in excess of 90.degree. C. and high hydrogen/ethylene ratios are employed in an attempt to obtain polymers having melt indices greater than 1.0 g/10 minutes. While the amount of ethane produced by this hydrogenation is at first relatively minor, this gaseous material gradually accumulates in the reactor and replaces the reactive monomers present. This decrease in the amount of monomer in the reactor leads to a corresponding decrease in catalyst productivity.